1. Field of the Invention
This invention relates to an external laser light introducing device for making the adjustment of the optical axis of laser light introduced from the exterior of an optical apparatus.
2. Description of Related Art
Recently, in accordance with high-performance and low-cost properties of laser light sources, optical apparatuses using laser light have increased in number. For example, in the field of semiconductor manufacturing apparatuses, it is common practice to apply a process technique using laser light in an ultraviolet region, from an excimer laser, in order to increase the integration density of a semiconductor. In the field of microscopes, coherent and monochromatic properties of a laser are utilized and thereby the image of higher resolution and contrast than in the case of conventional white light illumination is obtained. Consequently, scanning laser microscopes are being popularized.
In the case where the laser light source is used in combination with such an optical apparatus, an external laser light introducing device becomes necessary which permits optical axis alignment that laser light from the laser light source is aligned with the optical axis of the optical apparatus. In particular, when it is impossible to achieve alignment by moving the laser light source itself as in the case where the laser light source is large or an oscillation condition is very delicate, the optical axis alignment must be carried out inside the external laser light introducing device.
For example, Reference 1 described below, as shown in FIG. 2a, discloses an external laser light introducing device 220 constructed with a combination of a first movable mirror 221 and a second movable mirror 222, interposed between an excimer laser light source 200 and a projection exposure apparatus 210. In this conventional example, a laser position monitor 230 including a first half mirror 231, a second half mirror 232, and a light position detector 233 are provided inside the projection exposure apparatus 210. By using this laser position monitor 230, the angles of the first movable mirror 221 and the second movable mirror 222 are adjusted so that laser light 201 emitted from the excimer laser source 200 coincides with an optical axis 211 of the projection exposure apparatus 210, and thereby the optical axis alignment of the laser light 201 is carried out.
Reference 2 described below, as shown in FIG. 3, discloses an external laser light introducing device 320 that combines a movable lens 321, a fixed lens 322, and a movable plane-parallel plate 323, arranged on an optical axis 311 of an optical apparatus. In this conventional example, a laser light parallel control device 330 including a first beam splitter 331, a second beam splitter 332, a first light position detector 333, a second light position detector 334, and a lens driving circuit 335, interposed between the movable lens 321 and the fixed lens 322, performs the function of converging laser light 301 at the front focus position of the fixed lens 322 by moving the movable lens 321 along the optical axis 311. The fixed lens 322 performs the function of making the laser light 301 emerge parallel with the optical axis 311. A laser light position control device 336 including a third beam splitter 337, a third light position detector 338, and a plane-parallel plate driving circuit 339, provided behind the fixed lens 322, performs the function of making the laser light 301 emerging from the external laser light introducing device 320 coincide with optical axis 311 by adjusting an inclination relative to the optical axis 311 of the movable plane-parallel plate 323.
In the external laser light introducing device, the function of the focus position adjustment of the laser light is sometimes needed. Reference 3 described below, as shown in FIG. 4a, discloses a scanning laser microscope including a laser light source 400, a first beam expander 402, a microscope body 410, a focus position control optical system 420, and a confocal detector 430. In the microscope body 410, an objective lens 415 for magnifying and observing a specimen 413, a galvanomirror 412, and a second beam expander 414 connecting the position of the entrance pupil of the objective lens 415 and the galvanomirror 412 as a conjugate relation and fitting the diameter of the beam of laser light 401 to that of the entrance pupil of the objective lens 415 are arranged. The laser light 401 incident on the microscope body 410 along an optical axis 411, after being reflected by the galvanomirror 412 to pass through the second beam expander 414, enters the pupil of the objective lens 415 and is converged on the specimen 413. By oscillating the galvanomirror 412 to change the angle, the specimen 413 is scanned with the laser light 401 converged thereon. Reflected light, scattered light, or emitted light produced at the convergence position of the laser light 401 on the specimen 413 travels along the same optical path as the laser light 401 in a reverse direction, and a confocal image is detected by the confocal detector 430 including a beam splitter 431, a confocal lens 432, a confocal pinhole 433, and a photomultiplier tube 434. In the focus position control optical system 420 located on the optical path between the confocal detector 430 and the microscope body 410, a fixed convex lens 421 and a movable convex lens 422 constitute a nearly afocal system. The movable convex lens 422 can be moved along the optical axis to change the convergence or divergence of the laser light emerging from the focus position control optical system 420. Whereby, the height of convergence of the laser light 401 in the specimen 413 is changed and the focus position adjustment is carried out. The focus position control optical system 420 is also a kind of external laser light introducing device.
In a laser microscope having a plurality of laser light sources such as that disclosed in Reference 4 described below, when the laser light sources are switched or used at the same time, it is required that beams of laser light from the plurality of laser light sources to be focused on a specimen are converged at a point. In this case, on the basis of the light-collecting position of a laser light source as a reference, the optical axis alignment of laser light from each of the other laser light sources is carried out and at the same time, the focus position adjustment must be arbitrarily made.                Reference 1: Japanese Patent Kokai No. Hei 5-217844        Reference 2: Japanese Patent Kokai No. Hei 5-62210        Reference 3: Japanese Patent Kokai No. 2004-317676        Reference 4: Japanese Patent Kokai No. 2003-57554        
In conventional external laser light introducing devices, in order to make laser light coincide with the optical axis, it is necessary that the laser light is made to coincide with the optical axis at a reference point in the optical apparatus and the direction in which the laser light emerges at the reference point is made parallel with the optical axis.
For example, in the example of Reference 1, the light position detector 233 of the laser position monitor 230 corresponds to the reference point mentioned above. The direction in which the laser light emerges at the reference point is aligned with the optical axis in such a way that the return light of the laser light 201 caused by specular reflection from the second half mirror 232 of the laser position monitor 230 is superimposed with the laser light 201 in the proximity of the exit port of the excimer laser light source 200. However, when either the first movable mirror 221 or the second movable mirror 222 of the external laser light introducing device 220 is moved, both the position and the angle of the laser light 201 at the light position detector 233 are changed. Thus, the optical axis alignment needs alternate and repeated adjustments of the first movable mirror 221 and the second movable mirror 222.
The specific procedure of such adjustments is as follows. As illustrated in FIG. 2b, first, the first movable mirror 221 is adjusted so that the laser light 201 strikes the center of the second movable mirror 222 (Step 252), and the second movable mirror 222 is adjusted so that the laser light 201 strikes the center of the light position detector 233 (Step 253). Subsequently, a sheet of paper is put in the proximity of the exit port of the excimer laser light source 200 and the position of the return light specularly reflected by the second half mirror 232 of the laser position monitor 230 is viewed (Step 254) to ascertain whether the return light is axially aligned with the laser light 201 (Step 255). Here, if the return light is not axially aligned with the laser light 201, the first movable mirror 221 is adjusted so that the return light is superimposed with the laser light 201 (Step 256), and the procedure from Step 253 is repeated.
The example of Reference 2 is based on a major premise that the optical axis of the external laser light introducing device 320 and the optical axis 311 coincide with each other. However, it is very difficult to make invisible optical axes coincide. Common practice involves the operation that laser light for alignment is introduced along the optical axis 311 of the optical apparatus and the laser light for alignment is used to align the optical axis of the external laser light introducing device 320 with the optical axis 311.
Using FIGS. 4b-4d, the example of Reference 3 will be explained below. The movable convex lens 422 of the focus position control optical system 420, as shown in FIG. 4b, is set so that in the initial state, the laser light 401 emerging from the focus position control optical system becomes a parallel beam. In this case, the galvanomirror 412 of the microscope body is such that the beam of a proper diameter is incident thereon. In laser microscopes commercially available, however, a distance from the entrance port of the laser light to the galvanomirror is long, and when the movable convex lens 422 is moved away from the fixed convex lens 421 along the optical axis in order to shift upward the focusing position on the specimen, the laser light 401 emerging from the external laser light introducing device becomes convergent light and the beam diameter of the laser light 401 on the galvanomirror 412 becomes smaller than its proper value. Consequently, the laser light 401 ceases to meet the pupil diameter at the position of the entrance pupil of the objective lens, and thus the resolution of a confocal image obtained is deteriorated. On the other hand, when the movable convex lens 422 is moved toward the fixed convex lens 421 along the optical axis in order to shift downward the focusing position on the specimen, the laser light 401 emerging from the focus position control optical system becomes divergent light and the beam diameter of the laser light 401 on the galvanomirror 412 becomes larger than its proper value. Consequently, part of the laser light 401 is eclipsed at the galvanomirror 412 or the entrance pupil of the objective lens, and the amount of light is reduced. For example, when the wavelength of the laser light is 488 nm, the beam diameter of the laser light 401 emerging from the focus position control optical system is 3 mm, a distance from the focus position control optical system to the galvanomirror 412 is 1 m, and tolerances of variations of the beam diameter of the laser light 401 on the galvanomirror 412 are ±10%, it is seen from a simple calculation that the distance of the shift of the focusing position is limited to the degree of the depth of focus. That is, there is the problem that the focusing position cannot be substantially shifted.